1. (Field of the Invention)
The present invention relates to a hot film type air flow meter using a thin film type exothermic resistance element as an air flow rate detecting resistance element and a thin film type air flow temperature sensing resistance element, and more particularly to a hot film type air flow meter having a resin material flow meter main body suitably used as a suction air flow meter in an internal combustion engine for an automobile.
2. (Prior Art)
In recent years, a suction air flow meter has been used extensively for controlling a gasoline engine for an automobile. Recently, a hot film type air flow meter has been used as this suction air flow meter in the gasoline for the automobile. This hot film type air flow meter has a thin film type an air flow rate detecting resistance element as a main resistance element and also has a thin film type temperature sensing or temperature compensating resistance element as an auxiliary resistance element. Such a hot film type air flow meter is disclosed in, for example, Japanese Patent Laid-Open No. 236029/1985.
It is considered that the hot film type air flow meter having a thin film type resistance element such as an air flow rate detecting resistance element etc. enables the reliability to be improved and the manufacturing cost of the air flow rate detecting resistance elements and temperature sensing resistance elements as the hot film type resistance element to be reduced, respectively, and users in this technical field expect much from this hot film type air flow meter.
However, in the above-described prior art, no consideration is given to the radio interference resistance caused by electromagnetic waves or a considerably high frequency noise voltage of the hot film type air flow rate detecting resistance element etc. in the hot film type air flow meter. Consequently, in an automobile provided with the wireless equipment and various kinds of the oscillators as an electric equipment, the malfunction of the hot film type air flow meter occurs due to the radio guidance.
Especially, in recent years, a flow meter main body of the hot film type air flow meter in an engine for the automobile has been made of a resin material or plastics. This gives rise to a big problem concerning the radio interference resistance of the above-mentioned prior art hot film type air flow meter.
The hot film type air flow meter for use in a suction air flow meter in an engine for an automobile comprises a thin film type exothermic resistance element or resistor, for use of detecting the air flow rate of an air current or suction air, a thin film type temperature sensing resistance element or resistor for use of sensing the temperature of the suction air, a control module with an air flow state detecting circuit mounted thereon, and a flow meter main body constituting a part of a suction air passage in an engine.
The exothermic resistance element and the temperature sensing resistance element are assembled unitary with the control module. Since the control module is fixed to the flow meter main body which is molded integrally out a resin material. The exothermic resistance element and the temperature sensing resistance element are retained in the respective predetermined position in the suction air passage. The exothermic resistance element and the temperature sensing resistance element are projected greatly from the control module and retained in this state.
In the meantime, with a viewpoint to reducing the manufacturing cost of the hot film type air flow meter, the flow meter main body has been molded out of a resin material or plastics. Consequently, for example, when the wireless equipment is provided as an electric instrument in the automobile, the exothermic resistance element and the temperature sensing resistance element become exposed to a considerably high frequency electric field.
As a result, the exothermic resistance element and the temperature sensing resistance element work as antennas, so that an electromagnetic waves or a considerably high frequency noise voltage is introduced into the control module to cause the radio interference resistance of the hot film type air flow meter to decrease.
The characteristics of various radio interference resistance of the hot film type air flow meters having the exothermic resistance element and the temperature sensing resistance element will be described with reference to FIG. 4.
One example of the radio interference resistance of the conventional hot film type air flow meter has the characteristics curve a shown in FIG. 4, in which the radio interference resistance lowers greatly in the vicinity of, especially, the value of 470 MHz of frequency. This conventional hot film type air flow meter as it was remained substantially unendurable to practical use.
A hot film type air flow meter provided with a shield structure for the control module so as to improve the radio interference resistance thereof has also been proposed. To be concrete, the number of nodes between a shield base being disposed in the control module and the flow meter main body in the control module to ground is increased from one, which is the number referring to the conventional case of the hot film type air flow meter, to two to three.
The radio interference resistance characteristics of the node-increased hot film type air flow meter is as shown by curve b in FIG. 4. In this hot film type air flow meter, the radio interference resistance is improved to only a small extent. It is understood that it is difficult to obtain a satisfactory countermeasure by taking only such a step.